1. Field of the Invention
The present invention relates to a driving technology of an image sensor in an image capture apparatus.
2. Description of the Related Art
Recent image capture apparatuses such as digital video cameras and digital still cameras can record a moving image having a high resolution and high frame rate for a long time because of the improvement of the sensitivity of the image sensor, employment of advanced image processing, and an increase in the memory capacity.
Conventionally, a digital video camera or a digital still camera widely uses a contrast detection method as a method of detecting focus using an image sensor. In the contrast detection method, the sharpness of an object image formed by an image capture optical system is obtained by evaluating the output of a solid-state image sensor using a predetermined function, and the position of the image capture optical system on the optical axis is adjusted such that the function value takes an extreme value. To calculate the extreme value, the in-focus position needs to be passed once. For this reason, quite a long time is required until the in-focus state is obtained in principle.
On the other hand, there is known a technique of, in the image sensor, dividing a photodiode (to be referred to as a PD hereinafter) that exists in one pixel and receives light condensed by one microlens, thereby performing focus detection of a phase difference detection method on the image sensor plane. For example, in Japanese Patent Laid-Open No. 2001-83407, the PD in one pixel is divided into two PDs, thereby obtaining an output image (to be referred to as an AF image hereinafter) by a light beam from part of the exit pupil and an output image (to be referred to as a main image hereinafter) by a light beam from the entire exit pupil. Hence, the main image can be received while performing AF of the phase difference detection method using the AF image.
In the focus detection of the phase difference detection method, the defocus amount can be obtained. Hence, the time until in-focus can greatly be shortened as compared to the contract detection method.
In the technique described in Japanese Patent Laid-Open No. 2001-83407, however, if the AF image and the main image are received for each frame at the time of moving image recording or the like, the amount of data to process per unit time increases, and the power consumption increases. Since the calculation amount increases not only in the image sensor but also in the processing circuits connected to the subsequent stages, the amount of heat generated by the entire image capture apparatus increases. The image capture enable time shortens on account of image quality degradation due to heat generation and a need for device protection.
In addition, when the power consumption increases, the time until image capture prohibition shortens as the battery level decreases. This is particularly problematic when capturing a moving image at a high frame rate.